1. Field of the Invention
The present invention relates generally to bicycle frames, and more particularly to bicycle frames of composite materials formed without lugs.
2. Description of the Prior Art
Traditionally, bicycle frames have been composed of metal tubes, such as steel, welded into lugs at the tube joints. Non-metal materials for construction of various portions of bicycle frames have also been utilized. A number of manufacturers offer bicycle frames whose tubes are made of woven carbon fibers, for example. The benefits of using such a material in place of steel is its greater strength-to-weight ratio than steel. The difficulty arises, however, with the bonding of one tube of such material to another.
Most manufacturers have bonded carbon fiber tubing to cast and machined aluminum alloy connections to form tube joints. The problems associated with bonding carbon fiber to aluminum stem from the fact that the two different materials have extremely different structural properties. The drawbacks of bonding two dissimilar materials such as carbon fiber and aluminum for structural applications include:
1) The coefficients of thermal expansion of the two materials are substantially different. This can result in stress and/or strain being applied at the bonding surfaces which can weaken the bond and lead to joint failure.
2) The modulus of elasticity of aluminum is substantially lower than that of most composite materials, including commonly used carbon fiber materials. This means that relatively large, bulky connectors, or lugs, are required to provide the needed strength--at the cost of adding weight to the frame. This also means that the stiffness of the frame is discontinuous at the connections.
3) Aluminum readily corrodes when joined with carbon fiber due to the substantial difference in the galvanic corrosion potentials of the two materials.
4) Finally, the different fatigue characteristics of the two materials increases the potential for failure to occur at the connections.
Simply stated, these bonded frames have failed to optimize the structural benefits of the composite materials.
Attempts have been made to create bicycle frames with plastic tubes and injection molded connections. These connections are not suitable for a lightweight racing bicycle frame because there is no injection molded material which matches the stiffness and strength of steel or composite tubes required to withstand the rigorous abuse of racing conditions.
Monocoque frames exist which approach the optimal design for a composite bicycle frame. These frames are essentially one-piece molded structures. By "one-piece" it is meant that the frame is molded as a single complete unit. The primary drawbacks of this type of frame is the expense and complexity of tooling (including making the molds and related equipment, etc.), and the relative labor intensiveness of manufacture. A different mold is required for each size of frame. Pressure is required inside the tubes to expand the tubes such that they compress against the mold cavities. This pressure is usually applied by way of expandable rubber mandrels or bladders which involve additional expense and complexity. Pressure can also be applied by co-curing the composite with an internal foam core. The cure must be performed at elevated temperatures which cause the foam to expand and compress the composite material. The foam remains inside the tube as a permanent part of the structure. It is difficult, however, for the foam to apply sufficient pressure, with the result that the cured composite tube may contain voids (air pockets) which could result in premature delamination (separation) of the plies of composite material. For these reasons, bicycle frames manufactured by this process are generally more expensive and harder to produce in varied sizes and geometries than traditionally manufactured frames.
Finally, steel frames have been manufactured in "lugless" design, also referred to as fillet-brazed frames, for some time. As well, gussets have been incorporated into bicycle frame designs in the past. However, in each case, welding or soldering is the exclusive method used for joining tubes or gussets, and that method is clearly not employable when composite tubes are used.